America's top intelligence officers poring over new images of a suicide bombing in Syria unlike any other. Because they believe one of those suicide bombers was an American citizen a man from Florida who is he and have other Americans been turned into killers to. ABC senior justice correspondent Pierre Thomas says those new developments tonight. It was a powerful truck bomb in northern Syria. The al-Qaeda affiliated terrorists who produced this video of the attack say this American was a suicide bomber who drove the truck the video shows the truck being loaded with -- And driving toward its target. Government sources say the cat -- suspect is a US citizen from Florida. The first American to carry out a suicide attack in Syria. US officials are reaching out to the man's family and are investigating how he was recruited American fighters in Syria. A potentially imminent terror threat sparking concerns at the highest reaches of government adversary -- Civil war spilled across borders the capacity of battle hardened. Extremist groups to come after us only increase. At least seventy Americans are believed to a -- the thousands of foreign fighters is -- some have made their way back to the US sources say. And are under investigation -- surveillance and ABC news exclusive an FBI director James -- recently warned that America's who've gone to fight in the Syrian conflict. Might return home to formed sleeper cell for al-Qaeda. Can -- give us a sense of how active you are making sure. That you keep an eye on those people we are very active in civil war there is offering an opportunity for thousands. A foreign fighters to get training in the worst parts of terrorism they're gonna come back to Europe into the United States. A civil war in a land far away with potentially huge implications. For the US homeless. Pierre Thomas ABC news Washington.

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